Signal comparator fluid control circuit with reset

ABSTRACT

A fluid control circuit which compares the timing of two signals and shifts an output valve to an exhaust position in response to lack of synchronism between the compared signals. The circuit may be reenabled only by a reset signal pulse after the compared signals are in a similar condition. In one embodiment, the circuit (other than the output valve) uses only identical normally open three-way valves. In a modified form, some pairs of normally open valves are replaced by normally closed valves.

United States Patent Inventor Andrew B. Huntington Waterford, Mich.

App]. No. 30,038

Filed Apr. 20, 1970 Patented Sept. 28, 1971 Assignee Ron Operating Valve Company Detroit, Mich.

SIGNAL COMPARATOR FLUID CONTROL CIRCUIT WITH RESET [4 Claims, 10 Drawing Figs.

U.S. Cl 137/625.6, l37/596.14, 137/458 Int. Cl. Fl6k 11/02 Field 01 Search 137/458,

Reierences Cited UNITED STATES PATENTS 3,279,484 10/1966 Brinkel Primary Examiner-Henry T. Klinksiek Attorney-Harness, Dickey & Pierce ABSTRACT: A fluid control circuit which compares the timing of two signals and shifts an output valve to an exhaust position in response to lack of synchronism between the compared signals. The circuit may be reenabled only by a reset signal pulse after the compared signals are in a similar condition. In one embodiment, the circuit (other than the output valve) uses only identical normally open three-way valves. In a modified form, some pairs of normally open valves are replaced by normally closed valves.

PATENTEU SEF28 I97! 3,608,583

sum u or a SIGNAL COMPARATOR FLUID CONTROL CIRCUIT WITH RESET BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The invention relates to fluid control circuits which respond to lack of synchronism between two signals for disenabling an output line. Such circuits are useful for example as safety controls in the operation of clutch-and-brake mechanisms for presses or the like, in which the fluid control circuit for the mechanism passes through two valves which must act in synchronism in order for the circuit to be enabled. The signal comparator circuit senses lack of synchronism between these valves to disenable the main circuit, which can only be reenabled by being reset. Lack of synchronism may be caused for example by failure of one of the control valves to operate, and the disenablement will prevent injury to the operator or damage to the machine due to continued and uncontrolled application of fluid pressure.

2. DESCRIPTION OF THE PRIOR ART Examples of prior control systems of this type are found in U.S. Pat. Nos. 2,906,246 issued Sept. 29, i959 and 3,067,767 issued Dec. ll, 1962 to DiTirro et al. and in U.S. Pat. No. 3,170,484 issued Feb. 23, 1965 to Benz et al. These prior art devices have certain drawbacks which it is an object of the present invention to overcome. One limitation of the previous devices is that they depend for their operation on the relative counteraction of two opposing pressures which, when sufficiently unbalanced, cause the safety means to shift to its safety position. This principle of operation places definite limitations on the reliability and positiveness of action of the device. Furthermore, the unbalanced pressure which causes the shift to the safety position in the prior devices must counteract a spring force tending to maintain the safety valve in its normal open position. The present invention is of a positively acting, digital nature which does not depend on the unbalancing of opposing pressures but acts in a sharply defined on-ofi manner. The safety valve is a normally closed valve held open by pressure which counteracts the spring, so that this spring plus the supply port pressure on the valve will cause a positive shift to the closed or safety position. The normally closed safety valve of the present invention also eliminates a large number of sealing rings found on previous safety devices, thus reducing the frictional resistance to shifting.

Another disadvantage of prior art devices has been the possibility that a temporary loss in main air pressure will not necessarily lock out the controlled device in a safety condition. For example, in the DiTirro patents, if the electrical circuits for the solenoid-operated pilot valves are closed when the air pressure is temporarily lost, and if both main valves operate approximately simultaneously when pressure is restored, the safety valve will not shift to its safety position. The present invention prevents inadvertent reactivation of the system should a temporary loss of main pressure occur. In this event, the controlled device will be locked out and may only be reenabled upon application and removal of a reset signal.

A particular disadvantage of the system shown in Benz et al. U.S. Pat. No. 3,170,484, which is overcome by the invention, invention, is the fact that the control valve of that patent, when shifted to its safety position, does not immediately bleed down the pilot supply, and the motor is disenabled only after completion of the machine cycle and deenergization of the solenoids. According to the present invention, asynchronism between the sensed signals will result in immediate exhaustion of the output line regardless of the stage of the machine cycle.

BRIEF SUMMARY OF THE INVENTION In the illustrated embodiments of the invention, a pistonoperated three-way normally closed safety valve is placed in the output line to be controlled, although other types of safety valves could be used within the principles of the invention. Under ordinary conditions, assuming that the safety valve has been reset as described below, pressure is applied to the piston of this valve as long as the signals being compared act in synchronism, thereby holding the valve in its open position. Pressure on the piston is maintained by the output port of the safety valve through a restriction. A control line extends from the piston chamber but is ordinarily blocked by dumping valve means held in a closed position. The means for holding this dumping valve means closed comprises a triggering line which is ordinarily connected to exhaust.

The means for maintaining the triggering line in its exhausted condition comprises two piston operated three-way normally open triggering line control valves. The supply port of one of these valves is connected to a constant pressure source and the output port connected to the supply port of the second valve. The output port of the second valve is connected to the triggering line.

The piston chambers of the two triggering line control valves are connected to logic valve elements which in turn are controlled by the signals to be compared. More particularly, the piston chamber of the first triggering line control valve is connected to the output of AND" valve means so that this piston chamber will be pressurized when and only when both signals are being applied. The piston chamber of the second triggering line control valve is connected to the output of NOR" valve means so that this piston chamber is pressurized when and only when neither of the signals is being applied. Thus, the triggering line will be evacuated as long as both signals or neither signal is being applied. As soon as a situation occurs where only one signal is appliedand not the other, the triggering line will be pressurized, thus dumping the piston chamber of the safety valve and exhausting the output line.

Once the output line is exhausted, it can be repressurized only by applying and removing a reset signal after both signals being compared are in a similar condition. The reset signal is controlled by a selectively operated reset valve connected to the piston chambers of two valve means. The first of these is a piston-operated three-way normally open control line disconnect valve which is placed in the control line and, when actuated, will disconnect the piston chamber of the safety valve from the dumping valve means and connect it to exhaust. The reset signal is also connected through a restriction to repressurizing valve means. The supply port of this valve means is connected to a constant pressure source and the output is connected through a check valve to the supply port of the control line disconnect valve. Thus, when the reset signal is exhausted, the control line disconnect valve will immediately move to its open position, allowing the constant pressure source to repressurize the piston chamber of the safety valve before the repressurizing valve means returns to its closed position. In order for the repressurizing valve means to be effective, the dumping valve means must be in its closed position, that is, the triggering line must be exhausted. This in turn means that the compared signals must both be either present or absent.

In one embodiment of the invention, all the valves with the exception of the safety, reset and check valves are of an identical three-way normally open construction. However, under certain conditions some of the valve means may be replaced by normally closed valves. This is shown in a second embodiment of the invention wherein the four normally open valves comprising the AND valve means are replaced by a single normally closed valve, as are the two valves comprising the repressurizing valve means and the two valves comprising the dumping valve means. In this second embodiment, means are also shown for controlling the degree of signal discrepancy which will trigger the safety valve.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of a first embodiment of the invention using normally open valves, with the valves shown in their positions when both compared signals A and B, the reset signal and the output line are in their exhausted condition;

FIG. 2 is a cross-sectional view of a suitable normally open valve for use in the system;

FIG. 3 is a cross-sectional view of a typical three-way normally closed valve used as a safety valve;

FIG. 4 is a view similar to FIG. 1 but showing the valves in their positions when the reset signal is pressurized but with signals A and B not present;

FIG. 5 is a diagrammatic view similar to FIG. 4 but showing the positions of the valves when the reset signal is exhausted and the output line is therefore pressurized, signals A and B still not being present;

FIG. 6 is a view similar to the previous diagrammatic views but showing the positions of the valves when both signal pressures are applied approximately simultaneously;

FIG. 7 is a similar view but showing the positions of the valves when one and only one of the signal pressures is applied, the output thereby being shifted to exhaust;

FIG. 8 is a diagrammatic view of a second embodiment of the invention in which some of the sets of normally open valves are replaced by normally closed valves;

FIG. 8A is a partial schematic view of a modified form of the AND valve means of FIG. 8; and

FIG. 8B is a partial schematic view of an arrangement for controlling the degree of signal discrepancy which will operate the safety valve.

DESCRIPTION OF THE BREFERRED EMBODIMENTS Referring first to the embodiment of FIGS. 1-7, the safety valve is generally indicated at 11 and controls an output line 12 which may lead to the brake or clutch of a press. It should be understood that the principles of the invention encompass the use of shiftable mechanism 11 for purposes other than safety. As shown, valve 11 (FIG. 3) has a supply port 13, an output port 14, an exhaust port 15 and a spring 16 urging the valve toward a position connecting ports 14 and 15. A piston chamber 17 controlled by a port 18 (FIG. 1) is capable of being pressurized to counteract the spring force and the pressure from supply port 13 to shift the valve from its closed position as shown in FIG. 1 to an open position. A line 19 with a restriction 20 connects output line 12 to a control line generally indicated at 21 which supplies piston chamber 17. In the valve of FIG. 3, line 19 passes directly through the valve piston.

Control line 21 is ordinarily blocked by dumping valve means generally indicated at 22. In the embodiment of FIGS. 1-7, this means includes two three-way normally open valves 23 and 24. The output port 25 of valve 23 is connected to a piston chamber 26 of valve 24, and control line 21 is connected to the supply port 67 of valve 24. Therefore, with valve 23 in its normally open position, valve 24 will be held in its closed or blocking position, the pressure to chamber 26 being supplied from a constant pressure source connected to the supply port 27 of valve 23. Thus, any pressure from line 19 will hold valve 11 in its open position. It should be noted that in FIG. 1, valve 11 is in its closed or exhaust position because it has not yet been reset. FIGS. 5 and 6 show the dumping valve means effectively blocking the control line and therefore holding safety valve 1 1 in its open position.

Dumping valve means 22 is held in its closed position by the absence of pressure in a triggering line 28 connected to the piston chamber 30 of valve 23. The means for maintaining the triggering line in its exhausted position comprises two piston operated three-way normally open triggering line control valves 29 and 31. The supply port 32 of valve 29 is connected to a constant pressure source and the output port 33 connected to the supply port 34 of valve 31. The output port 35 of valve 31 is connected to the triggering line.

The piston chambers of the two triggering line control valves are connected to logic valve elements which in turn are controlled by the signals to be compared. More particularly, piston chamber 36 comprises valve 29 is connected to the out put of AND" valve means generally indicated at 37 so that chamber 36 will be pressurized when and only when both signals are being applied. The piston chamber 38 of valve 31 is connected to the output of NOR valve means generally indicated at 39 so that this piston chamber is pressurized when and only when neither of the signals is being applied.

Valve means 37 comprises four identical normally open three-way valves 41, 42, 43 and 44. The supply ports 45, 46 and 47 of valves 41, 42 and 44 respectively are connected to a constant pressure source. The output port 48 of valve 41 is connected to the piston chamber 49 of valve 42 and the working port 51 of valve 42 is connected to the supply port 52 of valve 43. The output port 53 of valve 44 is connected to the piston chamber 54 of valve 43 and the output port 55 of valve 43 is connected to the piston chamber 36 of valve 29.

Valve means 39 includes two identical normally open threeway valves 57 and 58. Supply port 59 of valve 57 is connected to a source of constant pressure and the output port 61 is connected to the supply port 62 of valve 58. The output port 63 of valve 58 is connected to chamber 38 of valve 31.

The means for resetting safety valve 11 comprises a control line disconnect valve 65 and repressurizing valve means generally indicated at 66. Valve 65 is a three-way normally open valve interposed in control line 21 between port 18 of safety valve piston chamber 17 and supply port 67 of valve 24. More particularly, supply port 68 of valve 65 is connected to port 67 and output port 69 is connected to port 18. Valve means 66 comprises two identical normally open three-way valves 71 and 72, supply port 73 of valve 71 being connected to a constant pressure source and output port 74 thereof connected to piston chamber port 75 of valve 72. Supply port 76 of valve 72 is connected to a constant pressure source and output port 77 thereof connected to the entrance to a check valve 78, the exit of this check valve being connected to control line 21 between port 68 of valve 65 and port 67 of valve 24.

Piston chamber port 79 of valve 71 is provided with a restriction 80. This port, together with piston chamber port 81 of valve 65, are connected to the output port 82 of a selectively operable three-way normally closed valve 83. The supply port 84 of this valve is connected to a source of constant pressure so that when the valve is closed, as shown 11 FIG. 1, valve 71 will be in its open position and therefore valve 72 in its closed position, valve 65 being in its open position.

A suitable normally open three-way valve for use in dumping valve means 22, as triggering line control valves 29 and 31 in AND valve means 37 and NOR" valve means 39, as control line disconnect valve 65 and in repressurizing valve means 66, is shown in FIG. 2. This valve has a housing 85 with end plates 86 and 87, a supply port 88, an output port 89 and an exhaust port 91. A valve disc 92 is urged by a spring 93 against a seat 94, disconnecting ports 89 and 91 and connecting ports 88 and 89. A piston 95 is actuatable by pressure at a piston chamber port 96 to shift disc 92 against a seat 97 so that ports 88 and 89 will be disconnected and ports 89 and 91 connected.

In operation, assuming an initial condition in which safety valve 11 and reset valve 83 are in their closed or exhaust positions and signals A and B are not present, the parts will be in the position shown in FIG. 1. Control line 21 and outlet line 12 will be exhausted. Triggering line 28 will be exhausted and dumping valve means 22 will be closed due to the position of valve 24.

Upon actuation of reset valve 83, valves 65 and 71 will be shifted to their closed positions (FIG. 4). This will shift valve 72 to its open position so that pressure will flow into portion 21a of control line 21 between valves 65 and 24, both of which remain closed. Safety valve 11 will remain in its closed or exhaust position so that output line 12 remains evacuated. It should be noted that since valve 65 connects safety valve chamber 17 to exhaust as long as reset valve 83 is actuated, output line 12 cannot be enabled with valve 83 held down.

Upon release of reset valve 83 the parts will arrive at their FIG. 5 position. Control line disconnect valve 65 will first return to its open position while repressurizing valve means 66 is still in its open position, as shown in FIG. 4, so that pressure from port 76 will flow through check valve 78 and valve 65 to chamber 17 of safety valve 11, opening this valve and pressurizing output line 12. After the piston chamber of valve 71 has been evacuated through restriction 80 sufficiently to permit this valve to shift into its open position as shown in FIG. 5 (thus shifting valve 72 to its closed position) valve 11 will remain open due to pressure passing from line 12 through restriction 20 to chamber 17. Dumping valve means 22 will remain closed since triggering line 28 is still evacuated.

It should be noted that since actuation and release of valve 83 is needed to reenable the circuit, the reset safety feature cannot be circumvented by tying down this valve.

If signals A and B are simultaneously applied after the parts reach their FIG. 5 position, they will be shifted to the position shown in FIG. 6. Application of signal A will shift valves 41 and 57 to their closed or exhaust positions. Shifting of valve 41 will cause valve 42 to shift to its open position. Application of signal B will cause valves 44 and 58 to shift to their closed or exhaust positions. Shifting of valve 44 will cause valve 43 to move to its open position. Pressurized air will thus pass through valves 42 and 43 to piston chamber 36 of valve 29, shifting this valve to its closed or exhaust position. Shifting of valve 58 will exhaust chamber 38 of valve 31 so that this valve will move to its open position. Triggering line 28 will thus remain evacuated by its connection through valve 31 to the exhaust port of valve 29. Dumping valve means 22 will thus remain in its closed (blocking) position so that safety valve 11 is held open and output line 12 is pressurized. Simultaneous removal of signals A and B will return the parts to their FIG. 5 position.

FIG. 7 illustrates the action of the circuit when one of the two signals, for example signal 8, is lost. Valves 44 and 58 will return to their open positions. This will have no effect on valve 31, but the pressurization of chamber 54 of valve 43 through open valve 44 will shift valve 43 to its exhaust position, thus shifting valve 29 to its open position. Pressure will pass through valves 29 and 31 to triggering line 28. When this occurs, dumping valve means 22 will open control line 21 to exhaust. This will happen by shifting of valve 23 to its exhaust position, thus opening valve 24. With output port 98 of valve 24 being connected to atmosphere, chamber 17 of safety valve 11 will be evacuated through valves 65 and 24. Valve 11 will thus shift to its exhaust or safety position, evacuating output line 12.

It should be noted that the presence of restriction 20 will insure shifting of safety valve 11 to its exhaust position, since control line 21 cannot be supplied with pressure from output line 12 at a sufficient rate to maintain the safety valve in its open position. The presence of a connected (with restriction 20) between output line 12 and control line 21 is of course necessary to maintain pressure in the control line when the safety valve is open and dumpingvalve means 22 is closed.

In order to reenable the circuit, dumping valve means 22 must be first closed by return of signals A and B to similar positions. For example, if signal B were to be reapplied, valve 44 would shift into its exhaust position, shifting valve 43 to its open position which in turn will shift valve 29 to its exhaust position to evacuate triggering line 28. This will open valve 23 and move valve 24 to its closed position, blocking control line 21. Alternatively, signal A could be released, in which case the parts would return to their FIG. 1 position which likewise closes dumping valve means 22.

Once this is done, application and removal of reset pressure at ports 79 and 81 will result in the parts going through their FIG. 4 to their FIG. 5 position. Once in this position, signals A and B may be applied and removed in synchronism without affecting the pressure in output line 12.

The operation of the circuit is also such that a temporary loss of air pressure will result in the output line being locked out in its safety or exhausted condition with reenablement possible only upon application and removal of the reset signal. Assuming for example that the parts arein their FIG. 5 position, with output line 12 pressurized, a loss of supply pressure will cause valve 24 of the dumping valve means to shift to its open position, exhausting control line 21. This will cause safety valve 11 to shift to its safety or exhaust position, evacuating output line 12. Valve 72 of repressurizing valve means 66 will shift to its open position but this will have no effect since there is no pressure. Upon supply pressure being restored, valve 72 will immediately return to its closed position. Valve 24 of the dumping valve means will also be returned to its closed position but control line 21 will remain evacuated and the parts will be in their FIG. 1 position. The only way to restore pressure to control line 21 and thereby shift safety valve 11 to its open position will be by application and removal of the reset signal as described above.

FIG. 8 illustrates a second embodiment of the invention which is similar to that previously described but in which some of the normally open valves are replaced by normally closed valves. More particularly, dumping valves means 22 of the previous embodiment, which comprised two normally open valves, is replaced by a single normally closed valve 101. AND valve means 37, which comprised four normally open valves, is replaced by a single normally closed valve 102. The two normally open valves which comprised repressurizing valve means 66 are replaced by a single normally closed valve 103.

The operation of the embodiment of FIG. 8 will be the same as that of the previous embodiment. FIG. 8 shows the parts in positions analogous to those shown in FIG. 1. That is, safety valve 11 is in its closed or exhaust position with dumping valve means 101 in its closed position but with control line 21 not pressurized. Actuation and release of reset valve 83 will pressurize control line 21 and open safety valve 11 so that output line 12 will be pressurized. The output line will remain pressurized as long as signals line A and B are applied and removed simultaneously. When signals are absent at A and B NOR" valve means 39 will maintain triggering line 28 evacuated by holding triggering line control valve 31 in its exhaust position. With both signals A and B applied, AND valve means 102 will hold valve 29 in its exhaust position and valve 31 will be moved to its open position, to maintain the evacuation of triggering line 28. The presence of one and only one of the two signals A and B will pressurize line 28 and cause dumping of line 21 to shift safety valve 11 to its exhaust position.

It should be understood that although FIG. 8 illustrates the dumping valve means, AND valve means and reset valve means all as comprising normally closed valves, any one or two of these valve means could comprise normally open valves as in the previous embodiment.

FIG. 8A illustrates an alternate form of AND" valve means utilizing two normally closed valves in series, one being actuated by signal A and the other by signal B. These valves are indicated at 201 and 202 and could replace valve 102 in cases where the signal capacity is limited. For example, should the signals be transmitted from relatively small openings leading from the actuating cylinders of a pair of main valves, signal A in FIG. 8 might be too weak to actuate valve 29. In such event, valves 201 and 202 could be used as an AND" valve means in which case the full supply pressure at supply port 203 of valve 201 would be instantly available for shifting valve 29 each time signals A and B are received.

FIG. 88 illustrates an alternative construction which may be placed in triggering line 28 if it is desired to have safety valve 11 to shift to its exhaust position only when there is a decided lack of synchronism between signals A and B. This alternative construction comprises a restriction 301 placed in line 28 together with a check valve 302 in parallel with restriction 301. Check valve 302 is so arranged as to block flow toward the dumping valve means but to permit free flow away from this valve means. With the construction shown, pressurization of triggering line 28 will not immediately shift dumping valve means (valve 101 in FIG. 8) to its open or dumping position since restriction 301 will delay this action in a selective manner.

What is claimed is:

1. A signal comparator fluid control circuit comprising a safety valve having a constantly pressurized supply port and at least one output port, means normally holding said valve in a closed position disconnecting said ports, a piston chamber pressurizable to move the safety valve to an open position, connecting said ports, a control line connected to said piston chamber, dumping valve means connected to said control line, means normally holding said dumping valve means in a position blocking said control line, a triggering line connected to said dumping valve means, the latter being responsive to pressure in said triggering line for exhausting said control line, a pair of three-way normally open triggering line control valves each of which has a piston chamber for moving the valve to a closed or exhaust position, a constant pressure source connected to the supply port of a first of said triggering line control valves, the output port of said first triggering line control valve being connected to the supply port of the second triggering line control valve, the output port of said second triggering line control valve being connected to said triggering line, AND" valve means connected to the piston chamber of one of said triggering line control valves, "NOR valve means connected to the piston chamber of the other triggering line control valve, and first and second signal lines each of which is connected to both said AND" and said NOR" valve means, whereby presence of neither or both of said signals will maintain said triggering line in an exhausted condition but the presence of one and only one of said signals will cause pressurization of said triggering line to evacuate said control line and shift said safety valve to its exhaust position.

2. The combination according to claim 1, further provided with a connection between the output line and said control line, a restriction being located in said connection.

3. The combination according to claim 2, said dumping valve means comprising a pair of piston-operated three-way normally open valves, the supply port of a first of said valves being connected to a source of constant pressure, the output port of said first valve being connected to the piston chamber of said second valve, the supply port of said second valve being connected to said control line, and the output port of said second valve being connected to the atmosphere.

4. The combination according to claim 2, said dumping valve means comprising a single normally closed three-way valve having a piston chamber connected to said triggering line, the supply port of said valve being connected to said control line and the output port connected to atmosphere.

5. The combination according to claim 2, said AND valve means comprising four identical normally open pistonoperated three-way valves, a constant pressure source connected to the supply port of a first of said valves, the output port of said first valve being connected to the piston chamber of the second valve, the supply port of said second valve being connected to a constant pressure source, the output port of said second valve being connected to the supply port of a third valve, said connection between said "AND valve means and said one triggering line control valve leading from the output port of said third valve, the fourth of said valves having its supply port connected to a constant pressure source and its output port connected to the piston chamber of said third valve, the piston chambers of said first and fourth valves being connected to the two signals.

6. The combination according to claim 2, said "AND valve means comprising a single normally closed three-way valve having a piston chamber connected to one signal and its supply port connected to the other signal.

7. The combination according to claim 2, said AND" valve means comprising a pair of normally closed three-way valves having piston chambers connected to the two signals, the supply port of the first of said valves being connected to a constant pressure source and the output port of said first valve Y connected to the supply port of the second valve.

8. The combination according to claim 1 further provided with means for resetting said safety valve, said resetting means comprising a normally gpen three-way control line disconnect valve connected in sar control line between said dumping valve means and the piston chamber of said safety valve, repressurizing valve means connected to said control line between said control line disconnect valve and said dumping valve means, and a pressurizable reset line connected to piston chambers of said control line disconnect valve and said repressurizing valve means, the line connected to the piston chamber of said repressurizing valve means having a restriction.

9. The combination according to claim 8, further provided with a check valve disposed between said repressurizing valve means and said control line and permitting flow only to the control line.

10. The combination according to claim 9, said repressurizing valve means comprising a pair of identical normally open three-way valves, the first of said valves having a constant pressure source connected to its supply port and its output port connected to the piston chamber of the second valve, and a constant pressure source connected to the supply port of said second valve.

11. The combination according to claim 10, further provided with a restricted connection between said output line and said control line.

12. The combination according to claim 9, said repressurizing valve means comprising a normally closed three-way valve having a constant pressure source connected to its supply port and its output port connected to said control line.

13. The combination according to claim 11, further provided with a restricted connection between said safety valve output line and said control line.

14. The combination according to claim 1, said safety valve comprising a three-way normally closed valve. 

1. A signal comparator fluid control circuit comprising a safety valve having a constantly pressurized supply port and at least one output port, means normally holding said valve in a closed position disconnecting said ports, a piston chamber pressurizable to move the safety valve to an open position, connecting said ports, a control line connected to said piston chamber, dumping valve means connected to said control line, means normally holding said dumping valve means in a position blocking said control line, a triggering line connected to said dumping valve means, the latter being responsive to pressure in said triggering line for exhausting said control line, a pair of three-way normally open triggering line control valves each of which has a piston chamber for moving the valve to a closed or exhaust position, a constant pressure source connected to the supply port of a first of said triggering line control valves, the output port of said first triggering line control valve being connected to the supply port of the second triggering line control valve, the output port of said second triggering line control valve being connected to said triggering line, ''''AND'''' valve means connected to the piston chamber of one of said triggering line control valves, ''''NOR'''' valve means connected to the piston chamber of the other triggering line control valve, and fIrst and second signal lines each of which is connected to both said ''''AND'''' and said ''''NOR'''' valve means, whereby presence of neither or both of said signals will maintain said triggering line in an exhausted condition but the presence of one and only one of said signals will cause pressurization of said triggering line to evacuate said control line and shift said safety valve to its exhaust position.
 2. The combination according to claim 1, further provided with a connection between the output line and said control line, a restriction being located in said connection.
 3. The combination according to claim 2, said dumping valve means comprising a pair of piston-operated three-way normally open valves, the supply port of a first of said valves being connected to a source of constant pressure, the output port of said first valve being connected to the piston chamber of said second valve, the supply port of said second valve being connected to said control line, and the output port of said second valve being connected to the atmosphere.
 4. The combination according to claim 2, said dumping valve means comprising a single normally closed three-way valve having a piston chamber connected to said triggering line, the supply port of said valve being connected to said control line and the output port connected to atmosphere.
 5. The combination according to claim 2, said ''''AND'''' valve means comprising four identical normally open piston-operated three-way valves, a constant pressure source connected to the supply port of a first of said valves, the output port of said first valve being connected to the piston chamber of the second valve, the supply port of said second valve being connected to a constant pressure source, the output port of said second valve being connected to the supply port of a third valve, said connection between said ''''AND'''' valve means and said one triggering line control valve leading from the output port of said third valve, the fourth of said valves having its supply port connected to a constant pressure source and its output port connected to the piston chamber of said third valve, the piston chambers of said first and fourth valves being connected to the two signals.
 6. The combination according to claim 2, said ''''AND'''' valve means comprising a single normally closed three-way valve having a piston chamber connected to one signal and its supply port connected to the other signal.
 7. The combination according to claim 2, said ''''ANDvalve means comprising a pair of normally closed three-way valves having piston chambers connected to the two signals, the supply port of the first of said valves being connected to a constant pressure source and the output port of said first valve connected to the supply port of the second valve.
 8. The combination according to claim 1, further provided with means for resetting said safety valve, said resetting means comprising a normally open three-way control line disconnect valve connected in said control line between said dumping valve means and the piston chamber of said safety valve, repressurizing valve means connected to said control line between said control line disconnect valve and said dumping valve means, and a pressurizable reset line connected to piston chambers of said control line disconnect valve and said repressurizing valve means, the line connected to the piston chamber of said repressurizing valve means having a restriction.
 9. The combination according to claim 8, further provided with a check valve disposed between said repressurizing valve means and said control line and permitting flow only to the control line.
 10. The combination according to claim 9, said repressurizing valve means comprising a pair of identical normally open three-way valves, the first of said valves having a constant pressure source connected to its supply port and its output port connected to the piston chamber of the second valve, and a constant presSure source connected to the supply port of said second valve.
 11. The combination according to claim 10, further provided with a restricted connection between said output line and said control line.
 12. The combination according to claim 9, said repressurizing valve means comprising a normally closed three-way valve having a constant pressure source connected to its supply port and its output port connected to said control line.
 13. The combination according to claim 11, further provided with a restricted connection between said safety valve output line and said control line.
 14. The combination according to claim 1, said safety valve comprising a three-way normally closed valve. 